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High AMP draw at idle

Captdave

Titanium
Joined
Sep 24, 2006
Location
Atlanta, GA
I upgraded my RPC from 10HP to 20 Hp to run a Mazak QT lathe. I bought it as a package and have been having some trouble with it. First of all when the idler is running and no load, it has a destinct "buzz" to it and has a 30 amp draw on one leg and 10-12 on the others. The builder claimes it's normal for idlers to have that much draw. Hummmm?

Using the same 20HP panel and hooking up the 10HP from my old RPC, I'm now getting the same "Buzz" as the 20HP motor and the same draw. The 10hp idler newer had a buzz when running with it's 10HP panel. The service legs are only showing 4-5 amps. How could this be normal?
 
RPCs draw real power and so-called "imaginary" power (correctly called "reactive" power) from the utility.

It is normal for an uncompensated RPC to present a low real power load to the utility. It is also normal for such an RPC to present a high reactive power to the utility.

The utility charges its residential customers based upon real power, only, but 1 amp on the real plane is the same as 1 amp on the imaginary plane, so your feeder could indeed be overloaded (by reason of the reactive power) when it is unloaded.

You can reduce the reactive power load on your feeder, and on the utility, by providing a source of reactive power, in the guise of a power factor correcting capacitor.

Uncompensated RPCs can have a power factor as low as 0.2, and some are as bad as 0.18.

(The ideal PF is 1.0, and you cannot achieve greater than 1.0, although when adding PFC caps you can move towards 1.0, then go past 1.0 and have the PF go down again).

So, in summary, it is quite normal for a simple RPC to have a high amp draw at idle, although you can significantly reduce this by providing a source of reactive power at the RPC.

You do NOT want to over-compensate the RPC, by providing too large a source of reactive power.

So, operate your RPC system, including its loads, close to maximum load, then add PFC caps until the amp draw is significantly reduced, then stop.

As a cross-check, see what the difference between unloaded w/o PFC and fully loaded w/PFC is.

Alas, RPC systems are not perfect, and these can be tuned for one load point, only.
 
I personally think it is better to discuss reactive *current* as opposed to real current.

Reactive = out of phase, real = in phase.

I like to confine the term "power" to real power only, seems to make the concepts
easier to grasp for most folks.

I'm not quite sure of his current readings though. I would expect to see the 10+ amps
on two of the legs, the ones supplied by the utility. When idling that would be nearly
all reactive current.

However the single 30 amp number, which I figure has to be on the manufactured
leg, does not make sense if the converter is simply idling and not running load motors.

It indicates to me that maybe something is wired wrong, or as you suggest, overcompensated?

Could there be a start circuit that is not dropping out, perhaps?

Jim
 
The 30A could be due to compensation caps.....

My Arco draws less from the mains than shows up in the compensation cap line, unloaded. It does NOT appear to be overcompensated. When actual power is drawn, the power factor improves, and the mains current comes up to more nearly equal the capacitor current.
 
I don't know the answer to your question but only relate my experience. I made a RPC with a 7.5 hp motor and it always made a nasty buzz until a load was added. Also, it drew more current from the utility with no machines running than it did with them added. This happened weather it was correctly balanced with caps or not. I changed the motor to a 15 hp idler and all the problems went away. Runs very nice now. Maybe try another idler? Doug
 
"I like to confine the term 'power' to real power only, seems to make the concepts
easier to grasp for most folks."

Perhaps, but reactive power is a "term of art", and is understood by those "skilled in the art" everywhere.

Isn't it better to emphasize the actual terms used by an industry, and explaining them where necessary, rather than concocting a new term, which term purports to make it easier to understand, yet it only adds to a "vocabulary" which is already overloaded with difficult to understand terms?
 
+1

The term "reactive power" is definitely well understood.

You are of course welcome to call it anything else that you so desire, even "Fred". But you will have to excuse us for not recognizing it until you explain!

BTW, the 30A reading was not linked with a "where it was measured" note..... it would be more useful to show the schematic, and point out the wire and schematic location where each current was measured.

I ASSUMED it was just on the manufactured leg, and INCLUDED the capacitor current. But that was an assumption, and may not be correct.

A more useful added measurement would be actual capacitor current, ALONG WITH the 3 line readings.
 
"Isn't it better to emphasize the actual terms used by an industry, and explaining them where necessary, rather than concocting a new term,"

Reactive current is NOT a 'new term' at all.

Trained in the art, yes, but most folks on this board, are not.

Conservation of energy is one of those fundamental concepts that folks tend to
intuitively grasp, easily. This is something I've found over the years.

Tell somebody their idling converter motor draws 14 amps, but it is current out
of phase with the voltage, and it really only burns 200 watts, they understand that.

Tell them that one kind of power is real, and another kind is imaginary, you get the
MEGO effect. Just does not sink it in my experience.

Draw the sine waves, here's the current, here's the voltage, see they don't happen
at the same time. The phase angle as a method to explain reactive current, works
very very well.

Look we're really talking pedagogical techniques here more than anything else. But
if you want to miss 90 percent of the folks you're trying to explain stuff to, start
talkng about the complex plane and the square root of negative one. Yep that'll
have them rolling in the aisles. :)

I like to keep discussion of "power" confined to real power. Just seems to work better.

Jim
 
Captdave,
How are things going with the rpc ?
I just bought a QT-20 and am trying to figure how to run it , the barn its in has only single phase. Am trying to get info on how much juce would actually be needed for light cuts . . . . the nameplate says 45kw 240/480 15hp I don't have the electrical manual.

David Lawrence
 
I sent the 20HP panel back last week and the replacement came yesterday. In the mean time I reinstalled a 10HP RPC and been using it. I just got back from a trip last night so will get on it in a day or two. One thing that you can do to lessen the power (amps) needed is to adjust your spindle accel/decal parameter. On my QT-8, it was set at .3 seconds and was drawing in the 70 amp range during large speed changes. I changed it to 5 seconds and it dropped into the low 30’s using a 10HP RPC.
 
"I like to keep discussion of 'power' confined to real power. Just seems to work better."

Total Power = Real Power + Apparent Power.

Real Power is that component of Total Power which is associated with the resistive part of the load, the so-called "heating power". This power produces work.

Apparent Power is that component of Total Power which is associated with the reactive part of the load, the so-called "imaginary power". This power doesn't produce work.

Both types of power indicate on your clamp meter.

On an unloaded idler, almost all indicated power is apparent power, not real power.
 
Peter,
The amp draw I was quoting was measured from the service side of the panel. Also, the lights no longer dim during speed changes which is a good thing for my power bill.
 
As far as "real" and "reactive" power........

If you doubt the "reality" of reactive power, ask the powerco........ They have to generate reactive power, and then they have to "ungenerate it" later in the cycle....... it heats wires, make transformers and generators bigger, etc. etc. etc.....

it's "real" enough...
 
"imaginary power."

Again from a pedagogical standpoint this is the proverbial turd in the punchbowl.
It is sure to be met with a scratching heads and quizzical looks by non-EE types.

Most folks will ask if it's quantified using imaginary numbers. Eleventeen, or Forty-twelve.

:)

The amp clamp meter measures amps. Some of them in-phase, some out of phase.

The in-phase ones are doing work. The out of phase ones, are not.

For the average HSM the power in question is indicated right on his watt-hour
meter. All in units of "real."

Jim
 








 
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